As a piezoelectric material, a lead-based piezoelectric ceramics such as lead titanate zirconate (hereinafter referred to as “PZT”) having a perovskite structure is generally used.
However, the PZT contains lead in the A site of the perovskite structure. Therefore, influence of lead on environment is a matter of concern. In order to address this problem, there has been proposed a piezoelectric material which uses a perovskite-type metal oxide containing no lead.
As a representative lead-free perovskite-type piezoelectric material, there is known barium titanate represented by general formula BaTiO3 (hereinafter referred to as “BTO”).
However, BTO has a phase transition temperature (orthorhombic-to-tetragonal phase transition temperature) in the vicinity of 0° C. to 10° C. Therefore, there has been a disadvantage that, depending on the use temperature, the piezoelectric property significantly varies. Various attempts have been made to eliminate this disadvantage.
For example, PTL 1 discloses a BTO ceramics in which the phase transition influence is reduced by doping Ca to BTO to shift the phase transition temperature to the lower temperature side. However, the piezoelectric property is degraded and the mechanical quality factor (Qm) is reduced as compared to those of BTO.
Further, PTL 2 discloses a BTO ceramics in which piezoelectricity is enhanced by doping Ca and Zr to BTO. However, the Curie temperature is 80° C. or less, and hence there has been a problem in that depolarization easily occurs under high-temperature environment. Further, the mechanical quality factor (Qm) is low.
Further, in general, pores are liable to be formed in the BTO ceramics, and hence there has been a problem in that the mechanical strength is not sufficient.
As described above, the BTO ceramics has had a problem in that the Qm and the mechanical strength are not sufficient.